Several different amplifier applications require an amplifier having a large gain-bandwidth product. For example, RF signals on optical fibers may require large gain-bandwidth product amplifiers that are highly linear. Some broadband fiber and RF communications applications may require large gain-bandwidth product amplifiers to provide high spectral efficiency. Software configurable communications systems may require an amplifier having a large gain-bandwidth product and a very wide operating bandwidth, which may span baseband frequencies to microwave frequencies. Baseband-to-microwave instrumentation may require an amplifier having a large gain-bandwidth product and a very wide operating bandwidth.
Distributed amplifiers (DAs) typically utilize multiple transconductance elements coupled together to provide an amplifier having a larger gain-bandwidth product than is possible with an amplifier using a single comparable transconductance element. A DA may have an input line of inductive elements or transmission line sections coupled in series and a parallel output line of inductive elements or transmission line sections coupled in series. The input and the output lines have corresponding taps that are coupled to the multiple transconductance elements, such that an input signal, which is applied to one end of the input line, propagates down the input line. As the input signal propagates down the input line, each successive transconductance element receives and amplifies the input signal to feed a corresponding tap into the output line. Each successive transconductance element adds to the amplified input signal.
In distributed amplifiers, input and output commensurate transmission lines are terminated with an RC network. The low frequency response is limited by the size of the capacitor. The low frequency response may be extended by employing an off chip parallel capacitor (DIE capacitor) through an inductive wire-bond. This can create both low frequency resonances and/or high frequency loading of the commensurate transmission line responses of the DA producing an undesirable response. While damping or de-Qing resistors may minimize the impact, it adds another off-chip component and increases module real-estate. Thus, there is a need for a DA to have a monolithic topology that does not rely on external components and which can resolve these parasitic effects.